SuperheatedsteamuidisedbedpaddydryingChaiyongTaechapairoja,*,IsaresDhuchakallayaa,SomchartSoponronnarita,SomboonWetchacamaa,SomkiatPrachayawarakornbaThermalTechnologyDivision,SchoolofEnergyandMaterials,KingMongkutsUniversityofTechnologyThonburi,Suksawat48Road,Thungkaru,Bangkok10140,ThailandbFacultyofEngineering,KingMongkutsUniversityofTechnologyThonburi,Suksawat48Road,Thungkaru,Bangkok10140,ThailandReceived18February2002；accepted24July2002AbstractFluidisedbedpaddydryingusingsuperheatedsteamisanewlyalternativeapproachinsteadofusingtheconventionalhotair.Themechanismofmasstransferforpaddydryinginarangeofinitialmoisturecontentbetween25%and44.5%d.b.isstronglycontrolledbyinternalmoisturemovementinsidethekernelandatwo-seriesexponentialequationissuitablyusedtoexplainitsmovement.Dryingparametersintheequationareafunctionoftemperatureandbeddepth.Forthepaddyquality,headriceyieldfromthesuperheatedsteamdryingismoresustainableandhashighervaluesthanthoseobtainedfromthehotairdrying,whereasthecolourofwhitericebecomesdarker,makingitpoorerquality.Thepercentageofwhitebellyissignicantlyaectedbytheinitialmoisturecontent.C2112002ElsevierScienceLtd.Allrightsreserved.Keywords:Dehydration；Grain；Headrice；Quality；Superheatedsteam；Whitebelly1.IntroductionPaddyisanimportantfoodcropintheworld.Thiscerealisprocessedintoseveralproducts,inadditiontoastaplefood.Beforebeingprocessed,freshly-harvestedpaddyneedstobedriedinordertoavoidthequalitydeteriorationbymicroorganismsandrespiration.Adryingmethodwhichisutilisedeectivelyfores-sentiallyhighmoisturegrains,forexample,paddy,parboiledrice,soybeanandmaize,isauidisationtechnique,inwhichairandsolidparticlesarerigorouslymixed.Ithasbeensuggestedthathighmoisturepaddyshouldbedriedquicklytoapproximate23%d.b.topreventtheyellowingofricekernelswhichiseasilyoc-curredathighmoisturelevelandthensubjectedtoambientairdryinginastoragebinuntilitsmoisturecontentisdownto16%d.b.(Soponronnarit&Prac-hayawarakorn,1994).SutherlandandGhaly(1990)undertookanintensiveinvestigationintothefeasibilityofusingahot-airuidised-beddryingtechniqueforpaddy.Theirexperimentsshowedthattheheadriceyieldrelatedtothenalmoisturecontent:theheadriceyieldwas5861%whenpaddywasdriedfrom28.2%to20.5%d.b.butwas1524%foranalmoisturecontentlowerthan20%d.b.Thedryingmediumwhichisusedforcarryingtheevaporatedwateranduidisingsolidparticlescouldbehotairorsuperheatedsteam.Superheated-steamdryingoersimportantadvantagesoverhotairdrying:(I)theenergysuppliedtothedryercanbereducedeconomi-callybyrecyclingtheexhauststeaminaclosedloop,(II)theenergyfromtheexhauststeamresultingfromtheevaporationofmoistureinsidethesolidscanberecov-eredandusedintheothersections,(III)environmentalpollutionorodouremissiontotheatmospherecanbeeliminatedsincedryingoccursinaclosedchamberwithoutair.Thegeneralcharacteristicsofthedryingprocesschangesignicantlywhensuperheatedsteamisusedinsteadofhotair.Themajordierencesinclude:(I)steamcondensationatthematerialsurfaceduringtheinitialdryingperiod,(II)higherdryingrateinsuper-heatedsteamthaninhotairataboveinversiontem-perature,thetemperatureatwhichtheevaporationrateofwaterintosteamandhotairbecomesequal(Yoshida&Hyodo,1963),(III)nogaslmresistancearoundthesolidsurfacetherebyincreasingthemasstransferrate.JournalofFoodEngineering58(2003)6773/locate/jfoodeng*Correspondingauthor.Tel./fax:+662-470-8663.E-mailaddress:chaiyong_(C.Taechapairoj).0260-8774/02/$-seefrontmatterC2112002ElsevierScienceLtd.Allrightsreserved.PII:S0260-8774(02)00335-7Theuseofahighsteamtemperaturemaycauseaseriousproblemforthermallysensitivefoodmaterials,suchasbrowning,discolourationandproteindenatur-ation.Insomecases,however,itchangesthetexturalpropertybenecially；forexample,shrimpdryingwithsuperheatedsteampresentedalowerdegreeofshrinkagethanthatusinghotair(Prachayawarakorn,Sopon-ronnarit,&Jaisut,2002).Thepurposeofthepresentworkwastomaketheparboiledricebyasuperheated-steamuidised-beddryer.Thedryingcharacteristicsofpaddywereinvesti-gated,alongwiththequalitiesintermsofheadriceyield,whitenessandwhitebellytobedetermined.2.MaterialandmethodsAbatchsuperheated-steamuidised-beddryerisschematicallyillustratedinFig.1.Itconsistedofvemaincomponents:acylindricalchamberwithaninnerdiameterof15cmandaheightof100cm,a13.5kWelectricalheaterforconvertingsaturated-steamtosu-perheated-steam,abackward-curvedbladecentrifugalfandrivenbya2.2kWmotor,areverseowcycloneandasmallboilerwitha31kg/hcapacityofgeneratingsteam.Aperforatedsheet,with10holespercm2,wasusedfordistributingthedryingmedium.Theratioofthediameterofdryertograindiameterdesignedwaslargeenoughtominimisethewalleect.However,thewalleectbecomesimportantwhentheratioislowerthan10(Geankoplis,1995).Superheated-steamtem-peraturewascontrolledbyaPIDcontrollerwithanaccuracyofC61C176C.Beforeusingsteamfordrying,hotairwasusedforwarmingupthesystemuntilthetem-peratureineverypartreachedthedesiredlevel.Then,theairwasreplacedbysteam.Thesteamgeneratorgeneratedthesaturated-steamat106kPa(absolutepressure)withthecorrespondingtemperatureof100C176C.Whenthesaturatedsteamwasowedthroughtheelectricalheater,additionalheatwassuppliedtoraisethesteamtemperatureuptothedesiredlevel.Itwassubsequentlypassedthroughtheuidised-beddryer.Afterthat,thesmalldustparticlesandtheimmaturegrainssuspendedintheexhauststeamwerecollectedinthecyclone.Finally,allthecleanedexhauststeamwasreusedagain.Thepaddywasrewettedtoadesiredmoisturecon-tentandthenkeptinacoolroomatthetemperatureof35C176Cfor57daystoensureuniformmoisturecontentthroughoutthekernels.Theexperimentalconditionsweresetupasfollows:initialmoisturecontentsof2545%d.b.,beddepthsof1015cm,superheatedsteamtemperaturesof150170C176Cataxedsupercialvelocityof3.1m/s.TemperaturesatdierentpositionsweremeasuredbyChromelAlumelthermocouples(TypeK)connectedtoadataloggerwithanaccuracyofC61C176C.Afterdrying,paddykernelswereslowlycooleddowntoambienttemperatureandkeptinapolypropylenebag.Then,theyweregentlyventilatedwithambientairuntiltheirmoisturecontentreached16%d.b.Finally,a300gsamplewaskeptinasealplasticbagfor2weeksbeforetestingforheadriceyield,whitenessandwhitebelly.Themoisturecontentofpaddywasdeterminedbyanelectricalairovenatatemperature103C176Cfor72h,accordingtoAACCmethod,1995.Paddyqualitiesintermsofheadriceyieldandwhitenessweredeterminedquantitativelyandcomparedtothereferencesample(paddydriedbytheambientair).ThemethodsfollowedtheguidelineoftheMinistryofAgriculturalandCo-operatives,Thailand.Headriceisdenedasmilledricehavingakernellengthatleast75%ofitsoriginallength.Theheadriceyieldwasdenedasthemassofwhitericethatremainsasheadriceaftercompletemillingdividedbythemassofpaddysample.Fromtheexperiments,theheadriceyieldoffreshpaddysamplesobtainedeachtimewasdierentanditwasthusdiculttocomparethepaddyqualityobtainedfromdierentdryingconditions.Tomakeaclearcomparison,theheadricequalitywasthereforerepre-sentedasarelativeheadriceyield.TherelativeheadriceFig.1.Aschematicdiagramofthesuperheated-steamuidisedbeddryer:(1)uidisedbeddryer,(2)heater,(3)fan,(4)cyclone,(5)boilerand(6)bypassline.68C.Taechapairojetal./JournalofFoodEngineering58(2003)6773yieldisdenedastheratioofheadriceobtainedfromarticialdryingtothatfromthenaturalairdrying.Inadditiontotheheadriceyield,theothertwoqualitiesofpaddy,translucence,whichwasdirectlymeasuredbywhiteness,andwhitebelly,werealsoex-amined.Theopaquewhitespotatthepaddykernelcanbelooselydividedintothewhitebellyandthechalkygrain.Thewhitebellyarisesfromincompleteorpar-tiallygelatinisedpaddy,whilethechalkygrainhasitsowncharacteristicdependinguponthegenotypeofpaddyandcanstillexistinthegrainevenwithcompletegelatinisation.ThewhitericesamplesweremeasuredforthecolourbyusingaKettdigitalwhitenessmeter(ModelC-300).Beforemeasuringthecolourofasam-ple,thewhitenessmetermustbecalibratedwithawhitecolouredreference,havingastandardvalueof86.3.Throughoutthiswork,thecolourofthesamplewasrepresentedintermsoftherelativewhiteness,whichwasdenedasthewhitenessofthesamplefromthearticialdryingdividedbythatfromnaturaldrying.3.Resultsanddiscussion3.1.FluidisationconditionsAtrst,theminimumvelocityrequiredforuidisinginthesuperheatedsteamwasdeterminedbymeasuringthepressuredrop.TheresultsareshowninFig.2forbeddepthsof10,12.5and15cm,indicatingthattheminimumsupercialvelocityforuidisationUmf,de-terminedfromthepointwherethepressuredropbeginstobeconstant,isindependentonthebeddepthwithavalueof2.6m/sforasteamtemperatureof150C176C.Thisminimumvelocityishigherthanthatusinghotairforwhichthevelocitywas1.65m/saspreviouslyreportedbySoponronnaritandPrachayawarakorn(1994).Therequiredhigheruidisingvelocityisprobablyduetothelowerdensityandviscosityofsuperheatedsteam.Thelowerphysicalpropertiesofsteamresultintheratiooftheverticalforceonthepaddybecominglowerforthesamesupercialvelocity.However,theforceactingonthepaddykernelintheoppositedirectioncanbeosetbyincreasingthesupercialsteamvelocity,thusrequiringahigherminimumuidisingvelocityforsteam.AscanbeseeninFig.2,belowtheminimumui-disingvelocity,thebedisastaticbed,correspondingtotheregionwherepressuredrophasalinearincreasewithsupercialvelocity.However,attheonsetofuidisa-tion,thepaddywaspartiallyuidisedanditrequiredavelocityatleast1.3timeshigherthantheminimumsu-percialuidisingvelocitytoachieveallparticlemove-ment.WhenapplyingtheErgunequation(1952),thecal-culatedminimumuidisationvelocitywas2.30m/satatemperature150C176C,slightlylowerthantheexperimen-tallydeterminedvalue.Thismightbebecauseoftheagglomerationofpaddykernelscausedbythesteamcondensation.Fig.3showstheminimumuidisingve-locityforairandsuperheatedsteam,andindicatestheslightdependenceofuidisingvelocityontemperature,butthestrongdependenceonthecharacteristicuidmedium,withthehigheruidisingvelocityforsteamoverawiderangeoftemperatures.Thesmalleectoftemperatureontheuidisingvelocitymightbebecauseofasmallchangeinthedensityandviscosityofthemediumsfortherangeoftemperatureconsidered.ThecharacteristicpropertiesofpaddyusedinthecalculationoftheuidisingvelocityareshowninTable1.Fig.2.Relationshipbetweenpressuredropandsupercialsuperheatedvelocityatdierentbedheight(superheatedsteamtemperature150C176C).Fig.3.Eectofdryingmediaontheuidisingvelocity.Table1PropertiesoflonggrainpaddyPhysicalpropertiesEquivalentdiameter(mm)3.5Truedensity(kg/m3)1414Void(emf)(Soponronnarit&Prachayawarakorn,1994)0.557Sphericity(；)0.68C.Taechapairojetal./JournalofFoodEngineering58(2003)6773693.2.DryingbehaviourFig.4showsthebehaviourofpaddykernelsdriedinsuperheatedsteamatdierenttemperaturesandbedheights.Inthersthalfaminute,thepaddykernelsadsorbasmallamountofthecondensedwaterandthus,theirtemperaturerapidlyincreasestothesaturationpointcorrespondingto100C176C.Ascanbeseeninthisgure,thesamplescangainwaterevenlyalthoughtheoperatingconditionssuchastemperatureandbeddeptharechanged.SuchresultsaredissimilartotheworkreportedbyTangandCenkowski(2000),whostudiedpotatodryinginabatchtraydryerusingsuperheatedsteam.Intheirwork,theinitialgaininmoisturecontentvariedwiththetemperature；thesamplescouldgainmoremoistureatlowersteamtemperatures.Apossiblereasonforthiscontradictioncanbethatusingahighsteamvelocityinthepresentwork(3m/sfortheui-disedbed)leadstothecombinationofhighheatandmasstransferrates.Thus,thelargeamountofcon-densedwaterexistingbetweenthekernelsevaporatesrapidly,insteadofdiusingintothegrains.FromFig.4,theinitialgainsinmoisturecontentforsamplesatdif-ferentconditionsareapproximately0.02d.b.Fig.5representsthecomparativedryingcharacter-isticsofpaddyinsuperheated-steamandhotair.Afterpassingthroughthecondensationstage,themoisturecontentreducesexponentiallywithdryingtime,indi-catingthatthemainresistancetomoisturemovementisintheinterior.Inthismoisturereductionstep,thedryingcurveisverysimilartothatfoundinthehotairdrying,butthetotaldryingtimeforsteamisshorterinspiteoftheexistenceofcondensationperiod.Thefastermoisturereductionisduetothehigherheattransferratewithsuperheated-steamatthistemperature.Thetotaldryingtimeforobtainingagivenmoisturecontentde-creaseswithincreaseinsteamtemperature,asexpected.Thisisconsistentwiththefundamentaldryingtheory,whichexpectsahigherheattransferuxtothebedofpaddywithhighertemperature.Besidestheeectoftemperature,thebeddepthor,intheotherwords,thetotalsurfaceareaofgrainsinthebedisanotherfactorthatreectstheamountofwaterevaporationfromthegrains；thickergrainbeddepthsleadtolowermoisturereductionratesforasinglegain,therebygivingalongerdryingtime.Thisisbecausetheheatsupplyisrathersmall,alongwiththepoorerheatconvection.AspreviouslyshowninFig.4,itclearlydemonstratedthattemperaturehadagreatercontribu-tionthanbeddepth.3.3.M